[roll] Roll fuchsia Implement async HTTP link checking in doc_checker

Optimize check_external_links in link_checker.rs by wrapping domain
validation futures in join_all, enabling fully concurrent HTTP/HTTPS
status checks across multiple external domains.

Derive Clone for LinkReference to support the concurrency changes.

Plumb external_links accumulation through check_metadata,
check_roadmap, check_supported_sys_config, and check_tools in
yaml.rs. This replaces the previous TODOs and routes all collected
external references to the async link checker during post_check.

Update all related unit tests to verify correct collection of
external links.

Original-Original-Bug: 500468244
Test: doc_checker_bin_test (compiles and runs the updated and new
concurrent link validation tests successfully).

TAG=agy
CONV=d3573b82-feee-45c7-8a5a-417977a6a533

Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1630354
Original-Original-Revision: a3a10fafe21e712b03ef8c640ead2f55a0fa295f
GitOrigin-RevId: f5b7d8d1490329203cf35c39df3a20e64ba7353d
Change-Id: I91bfde85cfda2da619805bf6c2392004e2e1eddc
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1632194
Cr-Commit-Position: refs/heads/main@{#193754}